This invention relates to a tyre pressure monitoring system, especially but not exclusively for use in monitoring the pressure within tyres of a road vehicle.
It is known to provide a sensor within each tyre of a vehicle which monitors the pressure within the tyre and a low-powered transmitter which transmits signals indicative of the pressure in the tyre to one or more receivers mounted to the vehicle. The receiver extracts the pressure signal from the received signals and generates a warning signal to the driver of the vehicle in the event that one or more of the sensors indicate that a tyre pressure is below a preset minimum safe level.
In one known system the vehicle is provided with a display which indicates each of the four tyres of the vehicle and their pressure. This can be achieved by placing a separate initiator next to each tyre which prompts the nearest transmitter to send out a signal. Initiating each transmitter in turn allows the receiver to identify which received signal corresponds to which tyre of the vehicle.
A disadvantage of such a system is that the cost of providing an initiator at each tyre, and the electrical cabling needed to connect the initiators to a central processing unit is much greater than that needed with a single receiver and no initiators.
In an alternative it is proposed that each transmitter will transmit along with the pressure signal an identity tag to a central receiver so that the receiver can distinguish between each of the four tyres according to their identity tags. The central receiver has access to a pre-stored set of identity tags which correspond to the transmitters of the four tyres of the vehicle and only listens to signals from transducers that have one of the four tags or codes. If a signal is received that has a different tag it can be ignored as it may be assumed to be associated with a tyre of a different vehicle.
This system is cheaper to implement and has a limited ability to “auto-learn” the identity of the transmitters which are fitted to the vehicle on initiation but can not easily cope with the situation where a tyre is removed from the vehicle and replaced by another. Either the stored list of codes must be updated or a tyre must be fitted using the original transmitter. The wheels can also not be rotated around the vehicle which is a common practice to ensure even wear of the front and rear road tyres.
In a refinement it is further known from EP0769395A1 to provide a system in which the identity of tyres of the vehicle is automatically learnt as identity tags are received. In the disclosed system, the order in which tags are received determines which transmitters are assumed to belong to the vehicle in a first in/first out basis. Subsequently received identity tags are held in a reserve list, again using a first in/first out basis. This is believed by the applicant to be unsatisfactory as it is very likely that the first received tags will include a tag from a tyre belonging to another vehicle. Furthermore, if no transmissions are received from a transmitter after a predetermined elapsed time it is removed from the list and immediately replaced with the first tag in the reserve list. This again is believed to be unsatisfactory.